Biology important questions

Plastids are double-membrane organelles of plant cells. Types: chloroplasts (green, with chlorophyll, in grana-stroma organisation), chromoplasts (coloured, in flowers/fruits) and leucoplasts (colourless, storage). A chloroplast has oute...

The Golgi complex (dictyosome) is a stack of 3-10 flattened, membrane-bound sacs (cisternae) arranged parallel and slightly curved, associated with small vesicles and vacuoles. It has two faces: the convex cis (forming) face towards the ...

The endoplasmic reticulum (ER) is a network of membrane-bound tubules, vesicles and cisternae in the cytoplasm connecting the nuclear membrane with the plasma membrane. Rough ER (RER) bears ribosomes and is important in protein synthesis...

The chloroplast is a green, double-membrane plastid. Between the two membranes and enclosing a matrix called the stroma, it has a system of flattened sacs, the thylakoids, stacked to form grana interconnected by stroma lamellae. The thylakoid membranes contain chlorophyll; the stroma contains enzymes, DNA, ribosomes and starch grains. Function: it is the site of photosynthesis - the light reactions occur on the grana and the dark (Calvin) reactions in the stroma, converting light energy into chemical energy (food).

Plastids are double-membrane-bound organelles found only in plant cells. Types: - Chloroplast (green, contains chlorophyll; site of photosynthesis). - Chromoplast (coloured other than green; gives colour to flowers and fruits). - Leucopl...

Prokaryotic cell Eukaryotic cell ------ Nucleus is not membrane-bound (nucleoid) True membrane-bound nucleus present Membrane-bound organelles absent Mitochondria, ER, Golgi, plastids etc. present Ribosomes are 70S Ribosomes are 80S (70S...

Steward's experiment (on carrot phloem cells growing into whole plants in culture) demonstrated cellular totipotency - that a single differentiated plant cell retains the full genetic capacity to regenerate into a complete plant.

RER (Rough Endoplasmic Reticulum) bears ribosomes on its surface and is involved in protein synthesis and transport. SER (Smooth Endoplasmic Reticulum) lacks ribosomes and functions in the synthesis of lipids and steroids and in detoxification.

The lysosome is called the suicidal bag of the cell because it contains powerful hydrolytic (digestive) enzymes that can, on rupture, digest the cell's own components (autolysis).

A prokaryotic cell is a primitive cell that lacks a true membrane-bound nucleus and membrane-bound organelles, its genetic material lying free as a nucleoid, e.g. bacteria and blue-green algae.

A cell wall is the rigid, non-living, outermost protective covering of plant cells, fungi and bacteria; in plants it is made mainly of cellulose and gives shape, support and protection.

Cyclosis is the streaming (rotational or circulatory) movement of cytoplasm within a living cell, which helps in distributing nutrients and organelles.

A coenocytic structure is a multinucleate mass of cytoplasm formed by repeated nuclear division without cell wall/cytoplasmic division, e.g. the hyphae of Rhizopus and Vaucheria.

Cellular totipotency is the inherent ability of a single living plant cell to divide, grow and develop into a complete organism under suitable conditions.

Plasmodesmata are fine cytoplasmic threads that pass through the pores in adjacent plant cell walls, connecting the protoplasts of neighbouring cells and allowing communication and transport between them.

The plasma membrane is selectively permeable and regulates the entry and exit of substances into and out of the cell, thereby maintaining the internal environment of the cell.

An incipient nucleus (nucleoid) is the false/primitive nucleus of a prokaryotic cell: a naked mass of circular DNA not bounded by a nuclear membrane, as in bacteria and blue-green algae.

a) Sporogony is the sexual phase of the life cycle of Plasmodium that takes place in the body of the female Anopheles mosquito. When the mosquito bites an infected human, it takes up microgametocytes and macrogametocytes along with the blood meal. Inside the mosquito's stomach the microgametocyte nucleus divides to form six to eight flagellated microgametes by a process called exflagellation, and one microgamete fuses with the macrogamete through a structure called the cone of reception to form a zygote. The zygote, now called an ookinete, penetrates the stomach wall and forms a cyst called the oocyst on its outer surface. Inside the oocyst the nucleus divides repeatedly to produce thousands of thread-like sporozoites, and when the oocyst ruptures these sporozoites migrate to the salivary glands of the mosquito, from where they are injected into a healthy human during the next bite, thereby continuing the life cycle.

b) The internal structure of the heart of frog consists of a large, funnel-shaped sinus venosus opening into the right auricle, a left auricle receiving oxygenated blood from the lungs and skin, both auricles emptying into a single, thick-walled ventricle, and a conus arteriosus with a spiral valve leading into the truncus arteriosus, which divides into the carotid, systemic and pulmocutaneous arches.

c) Wildlife extinction is mainly caused by the destruction and fragmentation of natural habitats through deforestation, urbanisation and the expansion of agricultural land, which deprives animals of their shelter and food. Excessive hunting and poaching for meat, skin, horns and other body parts, uncontrolled use of pesticides and other pollutants, the introduction of invasive alien species that outcompete native wildlife, and large-scale climatic change all add further pressure that pushes vulnerable species towards extinction.

d) The liver of the frog performs several important functions: it secretes bile, which is stored in the gall bladder and helps in the emulsification and digestion of fats in the small intestine. It also stores glycogen and releases glucose into the blood as required, helping regulate blood sugar levels. The liver detoxifies harmful substances absorbed from the gut, and it also plays a role in the breakdown of old red blood cells and in the metabolism of proteins.

e) Miller and Urey designed an apparatus consisting of two connected glass chambers, one containing water that could be boiled to produce water vapour, and the other containing a mixture of gases, methane, ammonia and hydrogen, in the ratio 2:1:2, meant to simulate the primitive atmosphere of the earth. Electric sparks were continuously passed through this gas mixture to simulate lightning, and the whole apparatus was run continuously for about a week, with the products being condensed and collected in a trap. Chemical analysis of the collected material showed the presence of simple amino acids such as glycine, alanine and aspartic acid. This result led them to conclude that, under the conditions believed to exist on the primitive earth, electric discharges from lightning could bring about the spontaneous formation of amino acids and other simple organic compounds from inorganic gases, supporting the biochemical theory of the origin of life.

f) Darwin's theory of natural selection has several drawbacks. It fails to explain the actual causes of the variations on which natural selection acts, since Darwin was not aware of the mechanism of heredity or the existence of genes and mutations. Darwin also treated all variations, whether inherited or acquired during an individual's lifetime, as equally significant for evolution, whereas it is now known that only germinal (heritable) variations, and not somatic ones, are passed on to offspring. Furthermore, the theory explains only the survival of the fittest organisms once variation already exists, but it does not explain the arrival, that is, the origin, of the fittest variation in the first place.

g) Class Mammalia is characterised by the presence of hair covering the body, and by the presence of mammary glands in females that secrete milk to nourish the young. Mammals are typically warm-blooded (homeothermic) with a four-chambered heart, they have differentiated teeth (heterodont dentition) set in sockets, a muscular diaphragm that separates the thoracic and abdominal cavities and assists breathing, and, in most species, they give birth to live young (viviparous), which are cared for by the parents after birth.

Six important characters of Phylum Mollusca: 1. Body is soft, unsegmented, and usually divided into head, muscular foot and visceral mass. 2. Body is covered by a soft fold of skin called the mantle, which often secretes a calcareous she...

Many fishes undertake regular migration for feeding and especially for breeding. Anadromous fishes (e.g. salmon, hilsa) migrate from the sea into rivers to spawn, while catadromous fishes (e.g. eel) migrate from fresh water to the sea to breed. They are guided by cues such as water temperature, currents, chemical signals (smell) and light. Migration helps them find suitable spawning grounds, food and favourable conditions, ensuring survival of the species.

General characters of class Mammalia: - Body covered with hair; skin has sweat and sebaceous glands. - Females possess mammary glands that secrete milk to nourish the young. - Warm-blooded (homeothermic); heart four-chambered; RBCs non-n...

Taxis is a directed locomotory movement of a free-living organism in response to an external stimulus. It is positive when the movement is towards the stimulus and negative when away. Types by stimulus include phototaxis (light), chemotaxis (chemicals), thermotaxis (temperature) and geotaxis (gravity), e.g. Euglena moving towards light. Taxis helps organisms seek favourable conditions (food, light) and avoid harmful ones.

Benefits of animal migration: - Enables animals to reach areas with abundant food and to escape food scarcity. - Provides favourable climate and shelter, escaping extreme cold or heat. - Ensures suitable breeding/spawning grounds for suc...

Feature Flatworms (Platyhelminthes) Round worms (Nematoda) --------- Body cavity Acoelomate (no body cavity) Pseudocoelomate (false coelom) Digestive tract Incomplete (single opening; mouth but no anus), or absent Complete (mouth and anu...

Fish migration is the periodic mass movement of fishes from one place to another for feeding, breeding or in response to environmental change. Types: - Diadromous migration between fresh and salt water: anadromous (sea to river to breed,...